In the operation of an electrochemical cell, it is critical that the anode and the cathode, both soaked by an electrolyte, be electrically isolated from each other so as to prevent shorting. This task is performed by a separator system which ensures good electrical insulation, while maintaining the desired ionic connection between the two electrodes. In the case of rechargeable cells, having a zinc anode and manganese dioxide cathode, the separator must also be capable of preventing the penetration of zinc dendrites or other such deposits which would also lead to shorting between anode and cathode. This problem occurs especially during the charging phase. The separator is formed from a sheet of the desired material which is rolled into a cylinder and inserted into the cell during manufacture.
While much research work has been done concerning the choice of an optimum separator system, less has been done on the problems relating to the sealing of the bottom edge of the separator to prevent shorting around the bottom of the separator, and the solutions so far have been less than ideal. This problem arises due to the above mentioned method of manufacturing a cell since the separator, in the cylindrical form, is open at both ends. The top of the separator, the end near the negative terminal of the cell, is sealed by a closure as is known in the art. The sealing of the bottom of the separator (i.e. proximate the positive terminal of the cell) is achieved in many cases by the use of hot-melts or sealants as discussed below.
In cylindrical cells in which the electrodes are arranged coaxially around each other, the separator takes a thin cylindrical form and is located between the anode and cathode. Such a cylinder, however, does not provide the desired separation and the bottom of the cell. Various solutions have been proposed to address this problem; however, most of these solutions have been found to have deficiencies. In one example, the separator is formed as an upright cylinder with a closed bottom. However, since the separator is generally made of a multi-layer sheet material, the required folding of the bottom of the cylinder results in wrinkling of the sheet and, therefore, weaknesses in the bottom which usually result in shorting problems.
Another solution is proposed in Swiss Pat. No. 669,479 in which a multi-layer separator material was used, and wherein a hot-melt material served to seal the bottom. In one embodiment of the invention, a sealant was applied over the bottom folded portion of the separator. In another embodiment, the lower edges of the cylindrical separator were folded back and a plastic disc with a stepped diameter was attached over the folded lower rim of the separator and a hot-melt was applied between the stepped portion of the disc and the interior of the separator cylinder. However, this method involves a complex manufacturing process and, in the result, a higher manufacturing cost.
U.S. Pat. No. 5,272,020 discloses a separator bottom seal where the separator extends to the bottom of the cell and where a hot-melt bead is applied as a seal.
U.S. Pat. No. 5,462,819 discloses a cell wherein a separator is placed into the cell, a hot-melt material is metered into the cell so that the hot melt material flows under the bottom edge of the separator which is then pushed down and seated in the hot-melt material as it cools. This results in a barrier formed at the bottom of the cell and on both sides of the separator.
In battery systems, increasing power and energy per unit volume and mass provide benefits. In rechargeable alkaline manganese dioxide-zinc cells intended for applications presently served by commercial primary (or non-rechargeable) cells, international standards specify maximum dimensions for the standard sizes. Manufacturers, therefore, strive for maximum performance within the limitations of the maximum dimensions. Minimizing the volume occupied by inert materials such as sealing components and hot-melts and maximizing the volume used for active materials, i.e. cathode, anode and electrolyte materials, results in increased performance. In rechargeable alkaline manganese dioxide-zinc cells, another constraint on cell volume relates to the requirement for sufficient void space to minimize any pressure build up resulting from gassing (i.e. the formation of hydrogen gas) during cycles of discharge and recharge.
Therefore, as can be seen, all of the patents mentioned above involve the use of a hot-melt adhesive or other sealant material that occupies space, which could otherwise be used for active materials and/or void space.
Further, when a hot-melt adhesive or other sealant is used to seal the bottom of the separator, such region is rendered impermeable to ions and, therefore, such region does not allow for any ionic connection between the anode and cathode. This results in reduced power capability of the cell and reduced efficiency at higher rates of discharge. When the cup and/or cups at the bottom of the separator are made of the same ion permeable material as the separator, more surface area is available and improved efficiency and performance is obtained at higher discharge rates even though the absorbent non-woven fibrous layers of the material are compressed.
The reduction or elimination of the hot-melt or other sealant provides a further benefit for commercial high speed production of cells in that electrolyte dispensed into the cathode/separator sub-assembly is absorbed more quickly, allowing faster machine speeds and/or less investment in inventory tables to provide sufficient delay time for electrolyte absorption.
U.S. Pat. No. 5,424,145 teaches a cell in which an improvement to the bottom seal is provided. In this reference, a cup is provided on the bottom of the cell which overlies the separator. The cup of this reference is comprised of one layer of polyethylene.
The present invention seeks to provide a cylindrical manganese dioxide-zinc cell with improved sealing of the bottom of the separator which provides more space for active materials and/or void space.